Cơ chế hình thành Carbon và cốc trên bề mặt xúc tác

One of the major problems related to the operation of heterogeneous catalysis is the catalyst loss of activity with timeonstream, i.e. ``deactivation. This process is both of chemical and physical nature and occurs simultaneously with the main reaction. Deactivation is inevitable, but it can be slowed or prevented and some of its consequences can be avoided. In the following, the causes of catalyst deactivation will be reviewed and the chemicophysical aspects related to the various deactivation processes will be discussed, along with mathematical description of the deactivation phenomena.

Catalysis Today 52 (1999) 165±181 Catalyst deactivation Pio Forzatti*, Luca Lietti Dipartimento di Chimica Industriale e Ingegneria Chimica ``G.Natta'', Politecnico di Milano, P.zza Leonardo da Vinci 32, 20133 Milan, Italy Abstract The fundamentals of catalyst deactivation are presented in this review The chemico-physical aspects concerning the various deactivation causes (i.e poisoning, sintering, coking, solid-state transformation, masking, etc.) have been analyzed and discussed, along with the mathematical description of the deactivation phenomena # 1999 Elsevier Science B.V All rights reserved Keywords: Catalyst deactivation; Catalyst poisoning; Catalyst sintering; Catalyst coking; Kinetics of catalyst deactivation Introduction One of the major problems related to the operation of heterogeneous catalysis is the catalyst loss of activity with time-on-stream, i.e ``deactivation'' This process is both of chemical and physical nature and occurs simultaneously with the main reaction Deactivation is inevitable, but it can be slowed or prevented and some of its consequences can be avoided In the following, the causes of catalyst deactivation will be reviewed and the chemico-physical aspects related to the various deactivation processes will be discussed, along with mathematical description of the deactivation phenomena 1.1 Chemical, physical and kinetic aspects of catalyst deactivation The knowledge of the chemical and physical aspects of catalyst deactivation is of pivotal impor*Corresponding author Tel.: +39-02-2399-3238; fax: +39-02-7063-8173 E-mail address: pio.forzatti@polimi.it (P Forzatti) tance for the design of deactivation-resistant catalysts, the operation of industrial chemical reactors, and the study of speci®c reactivating procedures Deactivation can occur by a number of different mechanisms, both chemical and physical in nature These are commonly divided into four classes, namely poisoning, coking or fouling, sintering and phase transformation Other mechanisms of deactivation include masking and loss of the active elements via volatilization, erosion and attrition In the following a brief description of the various deactivation mechanisms will be reported 1.1.1 Poisoning Chemical aspects of poisoning Poisoning is the loss of activity due to the strong chemisorption on the active sites of impurities present in the feed stream The adsorption of a basic compound onto an acid catalyst (e.g isomerization catalyst) is an example of poisoning A poison may act simply by blocking an active site (geometric effect), or may alter the adsorptivity of other species essentially by an electronic effect Poisons can also modify the chemical nature 0920-5861/99/$ ± see front matter # 1999 Elsevier Science B.V All rights reserved PII: S - ( 9 ) 0 - 166 P Forzatti, L Lietti / Catalysis Today 52 (1999) 165±181 of the active sites or result in the formation of new compounds (reconstruction) so that the catalyst performance is de®nitively altered Usually, a distinction is made between poisons and inhibitors [1] Poisons are usually substances whose interaction with the active sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface Poisons can be classi®ed as ``selective'' or ``nonselective'' In the latter case the catalyst surface sites are uniform to the poison, and accordingly the poison chemisorption occurs in a uniform manner As a result, the net activity of the surface is a linear function of the amount of poison chemisorbed In the case of ``selective'' poisoning, on the other hand, there is some distribution of the characteristics of the active sites (e.g the acid strength), and accordingly the strongest active sites will be poisoned ®rst This may lead to various relationships between catalyst activity and amount of poison chemisorbed Poisons can be also classi®ed as ``reversible'' or ``irreversible'' In the ®rst case, the poison is not too strongly adsorbed and accordingly regeneration of the catalyst usually occurs simply by poison removal from the feed This is the case, for example, of oxygencontaining compounds (e.g H2O and COx) for the ammonia synthesis catalysts These species hinder nitrogen adsorption, thus limiting the catalyst activity, but elimination of these compounds from the feed and reduction with hydrogen removes the adsorbed oxygen to leave the iron surface as it was before However, gross oxidation with oxygen leads to bulk changes which are not readily reversed: in this case the poison- ing is ``irreversible'', and irreversible damages are produced Upon poisoning the overall catalyst activity may be decreased without affecting the selectivity, but often the selectivity is affected, since some of the active sites are deactivated while others are practically unaffected This is the case of ``multifunctional'' catalysts, which have active sites of different nature that promote, simultaneously, different chemical transformations The Pt/Al2O3 reforming catalysts are typical examples: the metal participates in the hydrogenation± dehydrogenation reactions whereas alumina acts both as support and as acid catalyst for the isomerization and cracking reactions Hence basic nitrogen compounds adsorb on the alumina acid sites and reduce isomerization and cracking activity, but they have little effect on dehydrogenation activity ``Selective'' poisons are sometimes used intentionally to adjust the selectivity of a reaction: for example, the new Pt±Re/Al2O3 reforming catalysts are pretreated in the presence of low concentration of a sulfur compound to limit the very high hydrocracking activity Apparently, some very active sites that are responsible for hydrocracking are poisoned by S-compounds This treatment is known as ``tempering'' a catalyst [2] Table reports a list of the poisons typically encountered in some industrial catalytic processes In some cases, due to the very strong interaction existing between poisons and the active sites, poisons are effectively accumulated onto the catalytic surface and the number of active sites may be rapidly reduced Hence, poisons may be effective at very low levels: for instance, the methanation activity of Fe, Ni, Co and Ru Table Examples of poisons of industrial catalysts Process Catalyst Poison Ammonia synthesis Steam reforming Methanol synthesis, low-T CO shift Catalytic cracking CO hydrogenation Oxidation Automotive catalytic converters (oxidation of CO and HC, NO reduction) Methanol oxidation to formaldehyde Ethylene to ethylene oxide Many Fe Ni/Al2O3 Cu SiO2±Al2O3, zeolites Ni, Co, Fe V2 O5 Pt, Pd CO, CO2, H2O, C2H2, S, Bi, Se, Te, P H2S, As, HCl H2S, AsH3, PH3, HCl Organic bases, NH3, Na, heavy metals H2S, COS, As, HCl As Pb, P, Zn Ag Ag Transition metal oxides Fe, Ni, carbonyls C2H2 Pb, Hg, As, Zn P Forzatti, L Lietti / Catalysis Today 52 (1999) 165±181 Fig Effect of H2S poisoning on the methanation activity of various metals (Tˆ4008C, Pˆ100 kPa, feed: 4% CO, 96% H2 for Ni; 1% CO, 99% H2 for others) [3] catalysts is strongly reduced by H2S in the range 15± 100 ppb [3] (Fig 1) It follows that the analysis of poisoned catalysts may be complicated, being the content of poison of a fully deactivated catalyst as low as 0.1% (w/w) or less Extremely sensitive analysis is then mandatory, and since poisons usually accumulate on the catalyst surface, surface sensitive techniques are particularly useful Poisoning of metal-based catalysts Maxted [4] reported that for metal catalysts of groups VIII B (Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt) and I B (Cu, Ag, Au), typical poisons are molecules containing elements of groups V A (N, P, As, Sb) and VI A (O, S, Se, Te) The surface metal atoms active in the catalytic reactions can be depicted as involved in the chemisorption of the reactants (and of poisons as well) via their ``dangling orbitals'' Accordingly, any chemical species having a ``proper electronic con®guration'' (e.g unoccupied orbitals or unshared electron) or multiple bonds (e.g CO, ole®ns, acetylenes, etc.) can be considered as potential poisons Accordingly several molecules have been classi®ed as having 167 ``shielded'' or ``unshielded'' structures [4,5]: for example As in the form of arsine (AsH3), having a lone pair, is a strong poison for catalysts such as Pt in hydrogenation reaction, whereas no effect on catalytic activity is observed on the decomposition of H2O2, possibly because As under oxidizing conditions is present in the form of arsenate AsO3À Along similar lines the order of increasing poisoning activity for sulfur species, i.e H2S>SO2>SOÀ , can be explained Poisoning of metal oxide-based catalysts Metal oxide-based catalysts are generally more resistant than metal catalysts to deactivation by poisoning Acid catalysts (e.g cracking catalysts) are poisoned by basic materials (alkali metals or basic N-compounds) [6] Several studies have been reported in the literature concerning the effects of the nature (i.e Lewis versus Brùnsted) and strength of the acid sites and the basic character of the poison on the deactivation of acid catalysts [7±9] Oxide catalysts other than acid catalysts are also poisoned by several compounds, and often by Pb, Hg, As, Cd These compounds react with the catalyst active sites usually leading to a permanent transformation of the active sites which thus become inactive Preventing poisoning Poisoned catalyst can hardly be regenerated, and therefore the best method to reduce poisoning is to decrease to acceptable levels the poison content of the feed This is generally achieved by appropriate treatments of the feed, e.g catalytic hydrodesulphurization followed by H2S adsorption or absorption to remove S-compounds, methanation for the elimination of COx from the ammonia synthesis feed, adsorption over appropriate beds of solids to remove trace amounts of poisons (e.g ZnO for H2S, sulfured activated charcoal for Hg, alkalinized alumina for HCl) In several processes, e.g low-temperature shift, guard-beds (often constituted by the same catalytic material) are installed before the principal catalyst bed and effectively reduce the poisoning of the catalyst bed A review of a number of these methods can be found in [10] Another approach to prevent poisoning is to choose proper catalyst formulations and design For example, both Cu-based methanol synthesis and low-temperature shift catalysts are strongly poisoned by S-compounds In these catalysts signi®cant amounts of ZnO are present that effectively trap sulfur leading to the formation of ZnS The catalyst design (e.g surface 168 P Forzatti, L Lietti / Catalysis Today 52 (1999) 165±181 area, pore size distribution, pellet size) can also modify the poison resistance: these aspects will be brie¯y discussed in the next section Finally, it is noted that the operating conditions also affect the poison sensitivity of several catalysts: for example ppm sulfur in the feed poison a Ni/Al2O3 steam reforming catalyst working at 8008C, less than 0.01 ppm poison a catalyst working at 5008C, due to the increased strength of S adsorption 1.1.2 Coking Chemical aspects of coking For catalytic reactions involving hydrocarbons (or even carbon oxides) side reactions occur on the catalyst surface leading to the formation of carbonaceous residues (usually referred to as coke or carbon) which tend to physically cover the active surface Coke deposits may amount to 15% or even 20% (w/w) of the catalyst and accordingly they may deactivate the catalyst either by covering of the active sites, and by pore blocking Sometimes a distinction is made between coke and carbon The difference is however somewhat arbitrary: usually carbon is considered the product of CO disproportionation (2CO C‡CO2), whereas coke is referred to the material originated by decomposition (cracking) or condensation of hydrocarbons Mechanisms of carbon deposition and coke formation on metal catalysts have been detailed in several reviews [11±15]; they differ signi®cantly from those on oxide or sul®de catalysts [16] For instance, the mechanisms for carbon formation from carbon monoxide over Ni catalysts have been reviewed by Bartholomew [11] The rate-determining step is presumably the CO dissociation leading to the formation of various carbon forms, including adsorbed atomic carbon (C—), amorphous carbon (C˜), vermicular carbon (Cn), bulk Ni carbide (Cg), and crystalline, graphitic carbon (Cc) [17] The formation of such species depends on the operating conditions, catalyst formulation, etc In the case of the steam reforming of hydrocarbons on Ni-based catalysts, three different kinds of carbon or coke species were observed [18], i.e encapsulatedlike hydrocarbons (formed by slow polymerization of CnHm on Ni surface at temperatures lower than 5008C), ®lamentous or whisker-like carbon (produced by diffusion of C into Ni crystals, detachment of Ni from the support and growth of whiskers with Ni on top), and pyrolitic-type carbon (obtained by cracking of CnHm species at temperatures above 6008C and deposition of carbon precursors) The mechanism of coke formation on oxides and sul®des is rather complex but it can be roughly visualized as a kind of condensation±polymerization on the surface resulting in macromolecules having an empirical formula approaching CHx, in which x may vary between 0.5 and It has been suggested that the pathway to coke, starting from ole®ns or aromatics, may involve: (a) dehydrogenation to ole®ns; (b) ole®n polymerization, (c) ole®n cyclization to form substituted benzenes, and (d) formation of polynuclear aromatics from benzene [16] These mechanisms proceed via carbonium ions intermediates and accordingly they are catalyzed by Brùnsted acid sites The details of coke-forming reactions vary with the constituents of the reaction mixture, the operating conditions, and the catalyst used, but one can speculate that the reactive intermediates combine, rearrange and dehydrogenate into coke-type structures via carbonium ions-type reactions, as shown in Fig Carbonium ions can also crack to form small fragments that can further participate in the coke-forming process as hydrogen transfer agents The chemical nature of the carbonaceous deposits depends very much on how they are formed, the conditions of temperature and pressure, the age of the catalyst, the chemical nature of the feed and products formed Several authors pointed out a direct relationship between the amount of coke deposited and the aromatic and polynuclear aromatic content of the feed [19,20] Also, it has been reported that coke formation occurs more rapidly when a hydrogen acceptor, such as an ole®n, is present [21,22], in line with the hypothesis of a carbonium ion chemistry for coke formation Various analytical techniques have been used in order to characterize the nature, amount and distribution of coke deposits The chemical identity of the carbonaceous deposits has been extensively investigated by IR [23,24] Other techniques are well suited for this purpose, e.g UV±Vis, EPR, 13 C-NMR A short review of these methods has been recently reported [25] The amounts of coke deposited into the catalyst pores may be estimated by burning the coke with air and recording the weight changes via TG-DTA techniques and/or by monitoring the evolution of the combustion products CO2 and H2O P Forzatti, L Lietti / Catalysis Today 52 (1999) 165±181 Fig Carbonium ion mechanism for formation of higher aromatics from benzene and naphtalene [19] 169 170 P Forzatti, L Lietti / Catalysis Today 52 (1999) 165±181 Coke deposits may not be uniformly distributed in the catalyst pellets, and attempts were made to measure the coke concentration pro®les by several techniques, including controlled combustion, electron microscopy, H- and 129 Xe À NMR, XPS, AES [25,26] It appears that under certain conditions the coke pro®le is very non-uniform, with preferential deposition of carbon in the exterior of the particle The non-uniform coke deposition inside the catalyst pores may be related to the existence of intraparticle diffusional limitation, as reported by Levinter et al [27] It is noted that as coke accumulates within the catalyst pores, the effective diameter of the pores decreases, leading to an increase of the resistance to the transport of reactants and products in the pores If coke is concentrated near the pore mouth it will be more effective as a barrier than the same amount evenly distributed on the pore wall, and eventually pore blockage can occur [26±29] Preventing coke deposition In practice, the coke deposition may be controlled to a certain extent by using an optimal catalyst composition and an appropriate combination of process conditions During the reaction an equilibrium is reached between the rate of coke production and the rate of coke removal by gasifying agents (e.g H2, H2O and O2 that remove coke as CH4, CO and COx, respectively) so that steady-state conditions, corresponding to a certain level of coke present on the catalyst surface, are eventually reached Otherwise, if the rate of coke deposition is higher than that of coke removal, a suitable regeneration procedure must be applied For example, in hydro-desulfurization reactions the catalyst life is roughly proportional to the square of hydrogen partial pressure: hence, in spite of hydrogen cost, process equipment cost (high pressure) and operating costs (compression) still there remains a substantial economic incentive for operating at high H2 partial pressure Along similar lines in the catalytic reforming processes high hydrogen partial pressures are usually employed to limit the catalyst deactivation by carbonaceous deposits, and low hydrocarbon/steam ratios are typically employed in steam reforming over Ni catalysts In general, in many processes the gas mixture composition is kept as far as possible from conditions under which carbon formation is thermodynamically favored Obviously this is a necessary but not sufficient requirement in that carbon may form if the carbon forming reactions are inherently faster than the carbon-removal reactions The catalyst composition does also affect signi®cantly the coke deposition Promoters or additives that enhance the rate of gasi®cation of adsorbed carbon or coke precursors and/or depress the carbon-forming reactions minimize the content of carbon on the catalyst surface For this reason alkali metal ions, e.g potassium, are incorporated in several catalysts (e.g Ni-based steam reforming catalysts, Fe2O3± Cr2O3 dehydrogenating catalysts, etc.) Potassium has several effects: it neutralizes acid sites which would catalyze coke deposition via the carbonium ion mechanism previously mentioned, and catalyzes the gasi®cation of the adsorbed carbon deposits, thus providing an in situ route for catalyst regeneration Along similar lines, bimetallic Pt±Re/Al2O3 reforming catalysts are superior to Pt/Al2O3 in view of their greater resistance to deactivation by coking, which allows long activity (up to year) at relatively low H2 pressures, without regeneration 1.1.3 Sintering Sintering usually refers to the loss of active surface via structural modi®cation of the catalyst This is generally a thermally activated process and is physical in nature Sintering occurs both in supported metal catalysts and unsupported catalysts In the former case, reduction of the active surface area is provoked via agglomeration and coalescence of small metal crystallites into larger ones with lower surface-to-volume ratios Two different but quite general pictures have been proposed for sintering of supported metal catalysts, i.e the atomic migration and the crystallite migration models In the ®rst case, sintering occurs via escape of metal atoms from a crystallite, transport of these atoms across the surface of the support (or in the gasphase), and subsequent capture of the migrating atoms on collision with another metal crystallite Since larger crystallites are more stable (the metal±metal bond energies are often greater than the metal±support interaction), small crystallites diminish in size and the larger ones increase The second model visualizes sintering to occur via migration of the crystallites P Forzatti, L Lietti / Catalysis Today 52 (1999) 165±181 along the surface of the support, followed by collision and coalescence of two crystallites A number of different rate-limiting steps can potentially be identi®ed in either model, e.g the dissociation and emission of metal atoms or metal-containing molecules from metal crystallites; the adsorption and trapping of metal atoms or metal-containing molecules on the support surface; the diffusion of metal atoms, metal-containing molecules and/or metal crystallites across support surfaces; the metal particle spreading; the support surface wetting by metal particles; the metal particle nucleation; the coalescence of metal particles; the capture of atoms or molecules by metal particles; the metal atom vaporization and/or volatilization through volatile compounds As a matter of fact, sintering of supported metals involves complex physical and chemical phenomena that make the understanding of mechanistic aspects of the sintering a difficult task Experimental observations showed that sintering rates of supported metal catalysts are strongly affected by the temperature and to a lower extent by the atmosphere The effect of temperature and atmosphere can be easily derived from constant temperature± variable time data such as those reported in Fig 171 The ®gure shows two different regimes: a rapid, almost exponential loss of surface area during the initial stage and, later on, a slower (almost linear) loss These data may be consistent with a shift from crystalline migration at low temperatures to atomic migration at high temperatures [30] Contrasting data are available concerning the effect of the atmosphere on sintering For Pt-supported catalysts, several authors [31] reported that under oxidizing atmosphere the sintering is more severe than under inert or reducing atmosphere Bartholomew however observed that this is not a general case, since the rate of dispersion also depends on Pt loading (Fig 3) [32] These effects may be related to changes in surface structure due to adsorbed species such as H, O or OH in H2, O2 or H2O-containing atmospheres, respectively This points out the role of surface energy which depends on the gas composition and on the kinetics of the surface reactions Finally, the presence of strong metal±support interactions (SMSI) affect the spreading, wetting and redispersion of the supported metals: accordingly, because of the strong interaction of NiO with oxide supports, NiO/SiO2 is thermally more stable in air than Ni/SiO2 in H2 [32] Along similar lines, Pd stabilizes Pt in O2-containing atmospheres, possibly because of strong interactions of PdO with the oxide supports [33] Other factors affect the stability of a metal crystallite towards sintering, e.g shape and size of the crystallite [34], support roughness and pore size Fig Effects of H2 and O2 atmospheres and of metal loading on sintering rates of Pt/Al2O3 catalysts [32] 172 P Forzatti, L Lietti / Catalysis Today 52 (1999) 165±181 [35], impurities present in either the support or the metal Species such as carbon, oxygen, Ca, Ba, Ce or Ge may decrease metal atom mobility, while others such as Pb, Bi, Cl, F or S can increase the mobility Rare earth oxides such as CeO2 and La2O3 have been suggested to ``®x'' noble metal atoms in automotive exhaust converters due to a strong, localized chemical interaction [36±38] The effects of chlorides on the sintering of supported noble metal catalysts has been extensively investigated, since in several cases catalysts are prepared from chlorine-containing precursors (e.g H2PtCl6) or are treated with chlorine-containing compounds to maintain or enhance their acid properties The presence of chlorine either in the gas-phase or on the support favors the sintering of Pt [39] However, recently there has been an accumulation of convincing experimental evidences that Cl favors a process oppo- site to sintering, i.e redispersion [40] This process has been explained by either a physical splitting of the metal particles or to a spreading of metal monolayers over the surface The redispersion is of industrial importance in catalytic reforming over Pt/Al2O3 catalysts, where it has been observed that appropriate chlorine treatments in the presence of oxygen during the catalyst regeneration procedures may be useful for Pt redispersion This treatment, often termed as ``oxychlorination'', possibly involves the transport of metal oxide or oxychloride molecules through the vapor or along the surface Chlorides are also well known to cause severe sintering of Cu in Cu-based methanol synthesis and low-temperature shift catalysts (Fig 4) Metal oxide catalysts and supports are also affected by sintering, that is related to the coalescence and growth of the bulk oxide crystallites The process is Fig Temperature rise (A) and variation of catalyst activity (B, from laboratory data), Cu crystal size (C), Cl and S content (E and D, respectively) with reactor depth for an old charge of low-temperature shift catalyst in a commercial reactor [10] P Forzatti, L Lietti / Catalysis Today 52 (1999) 165±181 accompanied by an increase of the crystallite dimension leading to a decrease in the surface area and porosity Like for sintering of supported metal catalysts, also in this case the mechanisms leading to crystallites coalescence and growth are rather obscure In any case, the actual rate and the extent of sintering depends on many factors, including the metal oxide concerned, the initial crystallite size and the size distribution, the presence of additives that favor or promote sintering, the environment The key variable is temperature, so that operation at low temperatures greatly reduces the sintering rate Reaction atmosphere also affects sintering: water vapor, in particular, accelerates crystallization and structural change in oxide supports Accordingly, over high-surface area catalysts it is desirable to minimize the water vapor concentration at high temperatures during both operation and activation procedures as well The presence of speci®c additives is known to reduce the catalyst sintering For example BaO, CeO2, La2O3, SiO2 and ZrO2 improve the stability of g-alumina towards sintering [41±45], whereas Na2O enhances its sintering In addition to a decrease in the surface area, sintering may also lead to a decrease in the pore openings, and eventually the pores close completely making the active species inaccessible to the reactants 1.1.4 Solid-state transformation Solid-state transformation is a process of deactivation that can be viewed as an extreme form of sintering occurring at high temperatures and leading to the transformation of one crystalline phase into a different one These processes may involve both metal-supported catalysts and metal oxide catalysts as well In the ®rst case we can observe the incorporation of the metal into the support, e.g incorporation of metallic Ni into the Al2O3 support (at temperatures near 10008C) with formation of inactive nickel aluminate, or reaction of Rh2O3 with alumina (in automotive exhaust catalysts) to form inactive Rh2Al2O4 during high-temperature lean conditions In the case of metal oxide catalysts or supports the transformation of one crystalline phase into a different one can occur, like the conversion of g- into d-Al2O3 with a step-wise decrease in the internal surface area from about 150 m2/g to less than 50 m2/g Several of these transformations are limited by the rate of nucleation This process may occur due to the 173 Fig Effects of vanadia and tungsta loading on the surface areas of TiO2-supported V2O5-WO3 catalysts [46] presence of some foreign compounds in the lattice or even on the surface For example, V2O5 has been reported to favor the sintering of the TiO2-anatase support as well as the anatase-to-rutile transformation in TiO2-supported V2O5 catalysts On the other hand, WO3 effectively contrasts this phenomenon (Fig 5) [46] 1.1.5 Other mechanisms of deactivation Other mechanisms of deactivation include masking or pore blockage, caused e.g by the physical deposit of substances on the outer surface of the catalyst thus hindering the active sites from reactants In addition to the coke deposition already discussed, masking may occur during hydrotreating processes where metals (principally Ni and V) in the feedstock deposit on the catalyst external surface, or in the case of automotive exhaust converters by deposition of P (from lubricants) and Si compounds Certain catalysts may also suffer from loss of active phase This may occur via processes like volatilization, e.g Cu in the presence of Cl with formation of volatile CuCl2, or Ru under oxidizing atmosphere at elevated temperatures via the formation of volatile RuOx, or formation of volatile carbonyls by reaction of metals with CO [3] 174 P Forzatti, L Lietti / Catalysis Today 52 (1999) 165±181 Finally, loss of catalytic material due to attrition in moving or ¯uidized beds is a serious source of deactivation since the catalyst is continuously abraded away Accordingly the availability of attrition-resistant catalysts for ¯uid-bed catalytic cracking is extremely important since the process operates with regeneration and catalyst recycle Also, washcoat loss on monolith honeycomb catalysts may occur, especially when the gases are ¯owing at high linear velocities and/or when rapid changes in temperatures occur Indeed differences in thermal expansion between the washcoat and the honeycomb lead to a loss of bonding the surface Accordingly, if N0 is the number of active sites on a non-deactivated catalyst and Nt is the number of active sites at any stage of deactivation, the fraction of active sites is —ˆNt/N0 The goal is now to relate — with a Butt and Petersen [15] extended the Langmuir±Hinshelwood±Hougen±Watson (LHHW) kinetic approach to the description of systems of changing activity, and considered the dehydrogenation reaction of methyl-cyclohexane (A) to toluene (B) with formation of coke (C) according to the following scheme: Kinetics of catalyst deactivation A quantitative description of deactivating systems is essential in order to optimize the design and operation of catalytic processes, especially for fast deactivating systems The activity a of a deactivating catalyst is expressed according to the equation: a ˆ rar0 Y (1) where r0 is the initial rate of reaction (i.e., the rate of reaction of a fresh catalyst sample) and r is the rate of reaction measured after a determined time-on-stream) r0 is generally obtained by extrapolation to zero on a rate versus time-on-stream plot In general, the rate of reaction depends on the actual reaction conditions as well as on the activity, which is function of the previous catalyst history: r ˆ r…CY TY PY F F F Y a†X (2) According to the term coined by Szepe and Levenspiel [47], i.e separability, possibly the rate of reaction may be separated into two terms: a reaction kinetics dependency, which is time-independent, and an activity dependency, which is not: r ˆ r0 …CY TY PY F F F†r1 …a†X (3) Usually the separable factor r1(a) is simply taken as a normalized variable a (0 a 1) Since the activity of a catalyst (and hence the rate of reaction) is related to the population of the active sites on the surface, the catalyst deactivation can be considered as the decrease of the number of active sites on By considering the surface reaction A* D B* as the rate-determining step (kÀ2